The GGI consists of two pairs of high quality, low noise, matched accelerometers mounted on a block. Each of the accelerometers has an internal feedback loop for proper operation, and an external feedback loop for trim adjustment of the accelerometer scale factor and alignment of the accelerometer sensitive axis.
The normal configuration has the accelerometers mounted in opposing pairs, and equally spaced around the circumference of a circle, with their sensitive axes tangential to the circle. In use the block is rotated about a spin axis which is perpendicular to the plane of the circle, and passes through the centre of the circle. The outputs of the accelerometers of each pair are differenced and the difference signals are then combined. The overall effect is that the large common mode accelerometer output signals cancel to a high degree of precision, so that the residual differences which constitute the gradient signal are observable.
The accelerometers must be matched in their pairs so that the current/acceleration transfer function is matched in amplitude and phase at all frequencies of interest, to an accuracy of 1 part in 10.sup.10. The mismatch in accelerometer pairs is a result of the difference of the internal feedback closed loop errors, and thus the mismatch is also inversely proportional to the open loop gain. The existing external feedback scale factor adjustment can degrade, by an order of magnitude, the high frequency (&gt;1 Hz) lateral sensitivity for a 2% mismatch within the accelerometers. The influence of vertical acceleration on the accelerometers is an additional complicating factor.